Since it's a right triangle you can use the Pythagorean theorem or Trig (Soh Cah Toa).
I'll just use the Pythagorean theorem : a² + b² = c²
a = 9
b = x
c = 18
9² + x² = 18²
81 + x² = 324
x² = 324 - 81
x² = 243
x = √(243)
factor 243 to find a perfect square
x = √(3*81)
x = 9√(3)
